This application is a 371 application of PCT/JP00/01526, filed Mar. 14, 2000.
The present invention relates to a new metal-containing azo compound and an optical recording medium such as a tape, a disk, or a card for recording information by means of a light beam, particularly an optical recording medium in the form of a disk such as a compact disk (hereafter referred to also as CD) and a digital video disk (hereafter referred to also as DVD), in which the compound is used.
In recent years, with the spread of CDs, a write-once type optical disk satisfying the CD standard (hereafter referred to also as CD-R) has been developed and used. The CD-R is typically constructed by using a transparent resin substrate having a spiral groove on one surface thereof, applying thereon an organic dyestuff solution (i.e. an solution of an organic dyestuff) by means of a spin coater, then drying the solution to form a recording layer, disposing thereon a reflective layer formed by, for example sputtering gold, and disposing a protective layer formed by applying an ultraviolet-curable resin by means of a spin coater or the like and curing the resin (hereafter, the optical recording medium having the optical absorbing layer formed on the substrate by applying the organic dyestuff solution is referred to as an xe2x80x9capplication-typexe2x80x9d one).
Recordation on the CD-R is performed by forming pits on the recording layer in accordance with information, the pits being formed by changing (for example decomposing) the organic dyestuff and/or by using change of the interference structure of the recording layer due to the change of the organic dyestuff through irradiation of a laser beam having a wavelength of 780 nm from the substrate side to the recording layer. Reproduction of the information recorded on the CD-R is performed by detecting the changes in an amount of light reflected from the pits. In recording information on the CD-R by means of a CD-R writer or the like, servo characteristics which control rotation of a pick-up head along the groove of the substrate are of importance. Therefore, for the CD-R, it is necessary to obtain tracking error signals stably.
The tracking error signals are greatly affected especially by a level difference between recording layer parts on the groove portion and on the inter-groove portion of the substrate. Accordingly, upon producing the above-mentioned application-type CD-R, the following is carried out in order to ensure the tracking error signals. When an organic dyestuff solution is applied for forming a recording layer on a substrate, the solution fills preferentially the groove of the substrate. For this reason, a substrate having a deep groove is utilized to ensure a thickness of the recording layer and tracking error signals. However, as the depth of the groove in the substrate increases, a radial inclination in the groove depth is liable to take place, such as a groove depth decreasing from the inner periphery towards the outer periphery of the substrate, due to the influence of the temperature and the fluidity of the molten resin when forming the substrate. Further, an inclination in the recording layer thickness is liable to take place in the radial direction of the substrate due to a rise in viscosity of the solution caused by evaporation of the solvent during the application of the organic dyestuff solution.
Therefore, an attempt is made to suppress the change of the tracking error signals in the radial direction of the substrate to ensure a necessary level of the tracking error signals by precisely adjusting the application conditions such as a type of the solvent to be used for the organic dyestuff solution, a viscosity of the organic dyestuff solution, rotation control of the spin coater, and surrounding environment condition during the production, and also the drying condition and the like upon forming the recording layer.
Recently, in order to achieve a further higher density of information, a DVD is developed to which a laser having a wavelength around 650 nm is irradiated. The DVD is produced by forming pits with a track pitch of 0.74 xcexcm and the shortest pit length of 0.40 xcexcm on a substrate having a thickness of 0.6 mm by means of a molding apparatus, forming a reflective layer thereon, and further sticking another resin substrate having a thickness of 0.6 mm using an adhesive material such as an ultraviolet-curable resin in order to ensure the hardness of the disk, in the same manner as the CD. Like the CD-R, a writable DVD-R is being developed.
In view of compatibility with the DVD, the DVD-R must include pits which are formed to correspond to the information in a substrate having a narrow groove which has a track pitch of less than or equal to half of that in the CD-R and, in accordance therewith, has a narrower width. In this case, as the track pitch or the groove width of the substrate decreases, the inclination of the groove depth in the radial direction is further increased if the substrate has a deep groove. Accordingly, it is extremely difficult to ensure necessary tracking error signals by the conventionally performed adjustment of the application condition of the organic dyestuff solution and the drying condition.
In consideration of the above problems, an object of the present invention is to provide a metal-containing azo compound which is suited for a recording layer of an optical recording medium, and to provide an improved write-once type optical recording medium by using the compound which medium ensures more proper tracking error signals to be adapted to high density information recording and has various characteristics depending on the application of the medium,
The present invention provides a metal-containing azo compound represented by at least one formula which is selected from the following general formulae (a1), (a2), (a3) and (a4): 
wherein M(II) is a bivalent metal, each of X1 and X2 is a substituted or non-substituted residue that forms a monocyclic or polycyclic aromatic ring together with carbon atoms that are adjacent thereto at its both ends, each of Y1 and Y2 is a substituted or non-substituted residue that forms a nitrogen-containing aromatic heterocycle together with a nitrogen atom and carbon atoms that are adjacent thereto at its both ends, and a substituent when present in the residue is independently selected from the group consisting of a halogen atom, a substituted and non-substituted alkyl group, a substituted and non-substituted alkoxyl group, a substituted and non-substituted alkylthio group, a substituted and non-substituted aryl group, a substituted and non-substituted aryloxyl group, a substituted and non-substituted arylthio group, a nitro group, and a substituted and non-substituted amino group, and one residue is different from the other in at least one of a combination of X1 and X2 and a combination of Y1 and Y2; and the present invention provides an optical recording medium having a recording layer formed on a substrate, the recording layer allowing writing and/or reading information by a laser beam, wherein the recording layer comprises the metal-containing azo compound.
The metal-containing azo compound according to the present invention, which is represented by at least one formula of the above (a1), (a2), (a3) and (a4) is characterized in that two different azo compound ligands are bonded to the bivalent metal. It has been found that when such metal-containing azo compound is contained in a recording layer of an optical recording medium, characteristics brought by both azo compound ligands bonded to the metal appear moderately in the medium, and thus an optical recording medium of which overall performance is improved can be obtained. Hereafter, the present invention is explained in detail.